Systems for the measurement of a time period are well known and widely applied in sports time measurement. One of key elements is a camera, also referred to as a photofinish camera. This camera is special, for several reasons. First of all, It has to record very high speed images (“1-D” line or “2-D” image), currently in most systems from 100 to 2000 images per second. In addition thereto, each image has to be exactly time-tagged, with the race-time or day-time or other time-reference. In most cases the precision of this time recording is 1*10-3 s, where the resolution could be easily 1*10-6 s. This high frequency that is clearly above the standard 50 or 60 Hz for displaying on a screen, is a major problem in the field.
One such system is known from EP583441. This known system uses a photosensitive sensor comprising a first and a second coupled CCD device. Half of it is covered with a light-impermeable foil. Using this half sensor, the frequency of the timing signal is doubled to 100 Hz. A marking of the passage line is inserted into the resulting image. The marking may be moved with respect to the image forwards or backwards manually. The marking could also be a light pillar provided behind the passage line. The blocking of said light pillar is then an additional way of identifying the passage of an object or a person. A currently widely applied version of such light pillar is a photocell.
Another system and a calibration method are known from EP898249. The known system is provided with a reticle that is to overlie the image registered or to be registered by the camera. The line of passage is specified to be within this reticle. Processing means are foreseen to extract the relevant image portion corresponding to the reticle from the complete image. Its calibration method is aimed at positioning the reticle. In order to do this adequately, the camera must be aligned perfectly with the line of passage. This alignment is done manually and/or with the help of buttons. Once this is achieved, the camera is put into a spatial mode of registration. Herein, the camera reads at a first frequency of for instance 50 Hz the image signals, and have the resulting image including the line of passage displayed on a screen. The desired area is then selected. In the operation mode, merely the image within the reticle will be sent to the processor, and thus, the frequency can be increased.
A further such system is known from EP516449. This system also allows to select between different lines of an image. Signals corresponding to the line of interest are shifted at low speed, whereas signals corresponding the other lines are shifted at high speed. Therewith, the camera can serve as a slit camera in which data corresponding to a particular line is read out at an overall high speed.
Again a different system is known from WO92-15969. Time measurement in this system is not based on the viewing of the line, but on registration of competitive participants passing the passage line. Thereto, the participants carry an optical marking that includes a code that is specific to the participant. The code is preferably designed such that it can be read independent of the direction along which a participant passes the optical measurement system, for instance a laser scanner.
All of these prior art system suffer from the problem of calibrating the system in order to align the camera perfect with the passage line. The alignment is to be done on a desired point of the passage line, which is for instance its front part. In addition, one would like to adjust the optical device (e.g. lens) for illumination (iris), sharp picture (focus) and image size (zoom). It takes a lot of experience to adjust quickly and correctly and all these elements simultaneously, as the one setting typically affects another one slightly. Moreover, there may be a need to recalibrate the system in the course of a sports event. Light, zoom and focus typically need adjustment for a different type of a race and/or due to light changes.